A cover material of golf balls is an ionomer resin which is excellent in impact resistance, cut resistance and impact resilience. Two sorts of the ionomer resins, i.e. sodium neutralized ionomer resin and zinc neutralized ionomer resin, are commercially available under the trade names of "SURLYN", "HI-MILAN" and the like, and no other ionomer resins are available.
The sodium neutralized ionomer resin has superior impact, resilience in comparison with the zinc neutralized one. In order to improve impact resilience of golf balls, U.S. Pat. No. 3,819,768 proposes that the sodium neutralized ionomer resin and zinc neutralized ionomer resin are mixed. However, since ionomer resin available are only two sorts, as mentioned above, the modification by the mixing method are lacking in variety.
Japanese Kokai Publication (unexamined) 82768/1986 also proposes that the sodium neutralized ionomer resin, or a mixture of the sodium neutralized ionomer resin and the zinc neutralized ionomer resin, is additionally neutralized with other metal, such as lithium, potassium, cesium and rubidium to improve impact resilience. The ionomer resin is poor in reactivity in the additional neutralization. Accordingly, the ionomer resin used in the additional neutralization and an amount of it are so limited that it is difficult to obtain a high neutralizing degree.